1. Field of the Invention
The present invention relates to storage stability of aliphatic alkanals with a water content of greater than 300 ppm and to a method of improving the storage stability of these aliphatic alkanals, especially of substituted, sulfur-containing alkanals. It is known that members of this family of compounds are very reactive and that changes occur during storage.
A large number of undesired impurities arise from the formation of oligomers, polymers and condensation products, as well as from elimination reactions and oxidation and the subsequent reactions of the products formed.
Retardation of formation of undesired higher-molecular weight resultant products by means of suitable additives in a low dosage is described.
2. Description of Related Art
It is known that the formation of byproducts during the storage of alkanals can be influenced by alkaline or basic conditions. In an acidic environment the formation of oligomers, polymers and, especially, cyclic trimers (trioxans) are favored. In an alkaline environment the condensation of aldols is favored. DE-OS 2,095,267 discloses the use of triethanolamine or N,N-dimethylethanolamine for stabilizing aldehydes before cyclic trimerization, polymerization and autocondensation occur. Japanese patents JP 72 321 963 and JP 49 116017 disclose using alkylated anilines for stablizing aldehydes. U.S. Pat. No. 4,546,205 concerns use of a mixture of a pyridine and a phenol component as an aldehyde stablizing agent.
The stabilizers cited produce a basic environment and reduce the rate of the acid catalyzed reactions. The stabilizers may be toxic and are used in high concentrations, preferably up to 0.2%. Purification by distillation is often not possible because of the boiling points and the product properties.
PCT WO 93/13059 describes the stabilization of sulfur-substituted alkanals with a mixture of an amine component together with an oxygen-binding component such as phenols or acidic or unsaturated antioxidants such as ascorbic acid or betacarotene. However, this method is only effective if the water content of the alkanal is below 300 ppm, preferably below 100 ppm.
Such a water content can only be achieved at considerable expense under production conditions. A nitrogen atmosphere is customarily used in order to avoid autooxidation. None of the methods described for the stabilization of alkanals, however, meets the requirement that the stabilization takes place with non-toxic substances at low concentrations and that a water content of up to 2% can be tolerated.